@Entity
public class OrderUpdate extends BaseAuditedExternalIdItem {

    private Set<TankLineItem> _tankLineItems = new HashSet<TankLineItem>();

    @OneToMany(cascade={CascadeType.ALL}, mappedBy="orderUpdate")
    public Set<TankLineItem> getTankLineItems() {
        return _tankLineItems;
    }

    //...
}

@Entity
public class TankLineItem extends AbstractTankLineItem {

    private FuelLineItem _fuelLineItem;

    private OrderUpdate _orderUpdate;

    @ManyToOne (optional=false)
    @JoinColumn(name="orderUpdate_id")
    public OrderUpdate getOrderUpdate() {
        return _orderUpdate;
    }

    @OneToOne (cascade={CascadeType.ALL})
    public FuelLineItem getFuelLineItem() {
        return _fuelLineItem;
    }

    //...
}

@Entity
public class FuelLineItem extends AbstractFuelLineItem {
    //...
}

OrderUpdate ou1 = new OrderUpdate();

TankLineItem tli1 = new TankLineItem();
ou1.addTankLineItem(tli1);

TankLineItem tli2 = new TankLineItem();
ou1.addTankLineItem(tli2);

FuelLineItem fli1 = new FuelLineItem();
tli2.setFuelLineItem(fli1);

dao.saveOrUpdate(ou1);

//new session
Order order = (Order) _maxDao.findByExternalId(Order.class, "123");
assertNotNull("order", order);
assertEquals("update count", 1, order.getOrderUpdates().size());
OrderUpdate ou1 = order.getOrderUpdates().get(0);
Set<TankLineItem> tlis = ou1.getTankLineItems();
assertEquals("tank line item size", 2, tlis.size()); //!!exception here on load with second mapping//

@Entity
public class TankLineItem extends AbstractTankLineItem {

    private FuelLineItem _fuelLineItem;

    private OrderUpdate _orderUpdate;

    @ManyToOne (optional=false)
    @JoinColumn(name="orderUpdate_id")
    public OrderUpdate getOrderUpdate() {
        return _orderUpdate;
    }

    @OneToOne (cascade={CascadeType.ALL}, mappedBy="tankLineItem")
    public FuelLineItem getFuelLineItem() {
        return _fuelLineItem;
    }

    //...
}

@Entity
public class FuelLineItem extends AbstractFuelLineItem {
    private TankLineItem _tankLineItem;

    //@ManyToOne(optional=false)
    @OneToOne(optional=false)
    @JoinColumn(name="tankLineItem_id")
    public TankLineItem getTankLineItem() {
        return _tankLineItem;
    }

    //...
}

java.lang.NullPointerException
   at org.hibernate.engine.AssociationKey.hashCode(AssociationKey.java:39)
   at java.util.HashMap.hash(HashMap.java:264)
   at java.util.HashMap.put(HashMap.java:382)
   at java.util.HashSet.add(HashSet.java:194)
   at org.hibernate.engine.StatefulPersistenceContext.addNullProperty(StatefulPersistenceContext.java:1048)
   at org.hibernate.loader.Loader.registerNonExists(Loader.java:878)
   at org.hibernate.loader.Loader.getRowFromResultSet(Loader.java:548)
   at org.hibernate.loader.Loader.doQuery(Loader.java:665)
   at org.hibernate.loader.Loader.doQueryAndInitializeNonLazyCollections(Loader.java:221)
   at org.hibernate.loader.Loader.loadCollection(Loader.java:1699)
   at org.hibernate.loader.collection.CollectionLoader.initialize(CollectionLoader.java:71)
   at org.hibernate.persister.collection.AbstractCollectionPersister.initialize(AbstractCollectionPersister.java:490)
   at org.hibernate.event.def.DefaultInitializeCollectionEventListener.onInitializeCollection(DefaultInitializeCollectionEventListener.java:60)
   at com.connectria.kenan.dao.hibernate.AllEventListener.onInitializeCollection(AllEventListener.java:203)
   at org.hibernate.impl.SessionImpl.initializeCollection(SessionImpl.java:1438)
   at org.hibernate.collection.AbstractPersistentCollection.initialize(AbstractPersistentCollection.java:219)
   at org.hibernate.collection.AbstractPersistentCollection.read(AbstractPersistentCollection.java:81)
   at org.hibernate.collection.PersistentBag.size(PersistentBag.java:222)
   at com.connectria.kenan.model.NewModelDbTest$10.run(NewModelDbTest.java:379)
   at com.connectria.kenan.model.NewModelDbTest.runInSession(NewModelDbTest.java:92)
   at com.connectria.kenan.model.NewModelDbTest.testOrder(NewModelDbTest.java:371)
   at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
   at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
   at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
   at java.lang.reflect.Method.invoke(Method.java:585)
   at junit.framework.TestCase.runTest(TestCase.java:154)

package org.hibernate.engine;

import java.io.Serializable;

final class AssociationKey implements Serializable {
    private EntityKey ownerKey;
    private String propertyName;

    AssociationKey(EntityKey ownerKey, String propertyName) {
        this.ownerKey = ownerKey;
        this.propertyName = propertyName;
    }
    public boolean equals(Object that) {
        AssociationKey key = (AssociationKey) that;
        if (key == null || propertyName == null || ownerKey == null || key.propertyName == null || key.ownerKey == null) {
            System.out.println("This happens when you have a One-to-One with no matching One.");
            return false;
        }

        return (propertyName == key.propertyName || (propertyName != null && propertyName.equals(key.propertyName))) &&
                (ownerKey == key.ownerKey || (ownerKey != null && ownerKey.equals(key.ownerKey)));
    }
    public int hashCode() {
        return (ownerKey != null ? ownerKey.hashCode() : 0) + (propertyName != null ? propertyName.hashCode() : 0);
    }